1. Field of the Invention
The present invention relates to a developing apparatus adapted for use in an image forming apparatus of electrophotographic or electrostatic recording method such as a copying apparatus or a printer and capable of developing an electrostatic image on an image bearing member.
2. Related Background Art
Conventionally a developing apparatus shown in FIG. 5 is commonly employed for magnetic brush development utilizing a two-component developer.
The developing apparatus 1 is provided with a developing container 2 in which two developer bearing screws 6a, 6b are provided in parallel. The two-component developer contained in the developing container 2 is circulated by the above-mentioned screws 6a, 6b. The developing apparatus 1 is further provided with a developer bearing member 10 for bearing the developer in the developing container 2 toward a developing portion opposed a photosensitive drum 100 constituting the image bearing member. The developer bearing member 10 is usually composed of a cylindrical developing sleeve, in which a magnetic field generating means 11 constituted by a magnet roller is provided in an unrotating manner with respect to the rotation of the developing sleeve 10.
The developer in the developing container 2 is scooped up onto the developing sleeve 10 by a pole N2 of the magnet roller 11, and is borne, by the rotation of the developing sleeve 10, through poles S2, N3 and S1. In the course of bearing, the developer is regulated in thickness by a regulating blade 30 constituting developer regulating means and provided in noncontacting manner to the developing sleeve 10 in the vicinity of the pole S2, whereby a thin layer of the developer is formed on the developing sleeve 10. The pole S1 of the magnet roller 11, positioned at the developing portion, constitutes a main developing pole, and the developer caused to stand in the form of a brush by the pole S1 develops a latent image formed on the photosensitive drum 100. Subsequently the developer is removed from the developing sleeve 10 by the repulsive magnetic field of the poles N1, N2 and drops, thereby being returned into the developing container 2.
In such a conventional developing apparatus 1, however, the developer regulated by the regulating blade 30 remains in a large amount by the magnetic force of the poles S2 and N2 at the upstream side of the blade 30 with respect to the rotating direction of the developing sleeve 10, and such developer is subjected to a large pressure because the developer is continuously borne by the developing sleeve 10. As a result, in the developer remaining in the vicinity of the regulating blade 30, a strong force is applied between the magnetic carrier and the resinous toner to result in a phenomenon of embedding, into the resinous toner, of fine particles such as SiO.sub.2 and so on externally added to the toner particles. Also the shape of the resinous toner particles becomes rounder, by the elimination of sharp edges through collision with the magnetic carrier particles. Also after prolonged use, there results a so-called spent phenomenon that the resinous toner sticks firmly to the surface of the magnetic carrier and becomes unremovable.
With such a phenomenon, the amount of triboelectricity of the resinous toner varies with the time of use, thus leading to a variation in the image density or an increase in the mechanical sticking force of the resinous toner to the magnetic carrier or the photosensitive drum 100, eventually resulting in difficulty the image development or image transfer corresponding to the electric field and the local lack or unevenness of the toner. As a result, the image quality is extremely deteriorated in comparison with that in the initial stage.
FIG. 6 shows another example of the developing apparatus, in which, in order to prevent such remaining of the developer, the regulating blade 30 is positioned with an angle a, toward the downstream direction in the rotating direction of the developing sleeve 10 (counterclockwise in the drawing) from the tangential direction thereof, increased from 90.degree. to a range 150.degree. to 180.degree..
With such an arrangement of the regulating blade 30, the developer remaining after the regulating operation thereof advances along the regulating blade 30 and remains less in the upstream side, in the rotating direction of the developing blade 10, of the regulating blade 30. However, as the developer is continuously supplied by the developing sleeve 10 to the regulating blade 30, the superfluous developer gradually accumulates thereon and is pressurized. Consequently the developer deteriorates by the pressure after the prolonged use, and the deterioration of the developer cannot be improved significantly even by such configuration.